It is often necessary and desirable to measure the signals contained within a large frequency range via a swept-frequency measurement of electrical components and circuits to identify faults, verify performance, and determine characteristics of electrical components and circuits that may be referred to as a device under test (“DUT”). An example of a swept-frequency measurement is a spur search measurement, which is often listed among the tests that take the most time to execute. Examples of electrical components and circuits that often require swept-frequency measurements may include amplifiers, transmitters, modulators, and receivers, to name but a few examples.
Swept-frequency measurements typically involve a test device or synthetic test system tuning the local oscillator (“LO”) in a down-converter (or an up-converter, depending on the application) during the execution of the swept-frequency test. In either case, there are typically three known approaches to making such measurements. The first and simplest approach is to tune the frequency converter to a fixed frequency and then sample the analog signal in a digital-to-analog converter, or digitizer. A fast Fourier transform (“FFT”) may then be performed on the captured data, resulting in a set of frequency-dependent data within the bandwidth of the digitizer. While this may not technically be considered a swept-frequency measurement, in the sense that the frequency of the received signal is not changing, it nevertheless yields a measurement of the signal as a function of frequency, which is typically indistinguishable from a true swept-frequency measurement.
The second known approach to swept-frequency measurements may be referred to as a “stepped” mode of operation. In this case, a frequency converter is tuned to a fixed frequency and a measurement is taken. Once the measurement occurs, the frequency converter is then tuned to a new fixed frequency and another measurement is taken. This process continues until the desired frequency range has been swept. If a digitizer and FFT processor are utilized to make the measurements, each tuning step may result in measurements over the full bandwidth of the digitizer, as in the previous approach. These measurements may then be concatenated to yield a measurement over the desired frequency range. There are typically no timing difficulties associated with this approach and it is the one most often chosen for synthetic test systems, even though these types of synthetic test system measurements have additional overhead associated with them.
The third approach to swept-frequency measurements is often referred to as a “swept” mode of operation. In the “swept” mode, a frequency converter is tuned by a continuous ramp function, resulting in a measurement frequency that changes linearly as a function of time. During this type of sweep, the digitizer takes samples of the signal at regular time intervals. As a result, each sample contains a value that reflects the strength of the signal at one particular instant in time, and that particular instant in time may in turn be related to the measurement frequency. Utilizing this mode, no FFT is needed, because the samples from the digitizer are taken directly at the desired measurement frequencies. Unfortunately, this swept-frequency measurement mode has traditionally not been available to synthetic instruments.
As mentioned above, the frequency of measurements taken in “swept” mode is determined from the measurement timing. In order to accurately determine the measurement frequency in “swept” mode, four things should be known: 1) the time at which the frequency starts to sweep, 2) the rate at which the frequency is changing, 3) the time at which the data collection starts, and 4) the sample rate of the data. The second and fourth of these are generally predetermined from a knowledge of the system, but the other two are determined at the time of the measurement. The more accurately the sweep and data collection start times (or their relationship to each other) are determined, the more accurately the measurement frequencies may be aligned with the corresponding data.
In devices such as, for example, a spectrum analyzer, an internal system controller, such as a computer, that has direct hardware connections to the digitizer and down-converter, typically initiates the sweep operation. Synchronization during the measurement sweep is typically handled by hardware trigger signals that directly connect to the various measurement subsystems (such as, for example, receiver, filter, digitizer, etc.). In a synthetic instrument, however, this approach is difficult or impractical to implement and may add considerable complexity to the system because synthetic instrument test systems typically include multiple, independent, non-integrated instruments. As a result, direct hardware synchronization between these instruments is difficult and more complex than in traditional integrated instruments. As an example, the implementation of a spectrum analyzer utilizing synthetic test instruments generally requires a down-converter, a digitizer, and a controlling computer that are all implemented as separate standalone devices. While an integrated spectrum analyzer may synchronize its internal operations utilizing internal hardware signals, and thus achieve highly accurate swept-frequency measurements, this accuracy is more difficult to achieve using synthetic test instruments, due to their lack of synchronization.
In comparing a traditional spectrum analyzer and a synthetic analyzer, it is appreciated that both “swept” and “stepped” modes of swept-frequency measurements are possible in each type of analyzer. Additionally, in “stepped” mode, each analyzer moves the LO to the next frequency and allows the frequency to settle before the measurement is taken. If it is assumed that the local oscillators are identical in the two analyzers, then there should be no difference in the LO frequency settling time. However, there will be a difference in the overhead associated with executing the frequency change and subsequent data collection, because the spectrum analyzer utilizes internal hardware signaling to coordinate the two processes, while the synthetic analyzer does not. In typical implementations, this leads to slower “stepped” mode measurements as each instrument in the synthetic analyzer is stepped to a new frequency and a measurement is performed. While “swept” mode measurements are possible utilizing synthetic instruments, without the sweep synchronization, they are capable of achieving sufficient accuracy only at greatly reduced sweep rates, making them of questionable value.
Therefore, there is a need for a system and a method that allows synthetic instrument systems to perform “swept” and “stepped” mode measurements without requiring dedicated hardware triggering between the separate synthetic instruments.